This invention relates to materials for use at high temperatures. More particularly, this invention relates to materials designed for enhanced toughness at high temperatures. This invention also relates to methods for making such materials.
Materials with the capability to maintain adequate properties at extremely high temperatures are highly sought after for use in several widely varying applications, including, for example, space vehicles, turbine equipment for power generation plants and aircraft engines, and metal forming and glass blowing equipment. For instance, increasing the temperature of the combustion gases used to drive a gas turbine generally increases the potential efficiency with which the turbine can generate power. However, the alloys and protective coatings used to fabricate turbine components typically operate at or near their temperature limits in state-of-the-art turbine equipment, and even a modest increase in firing temperature of such a turbine would degrade the performance of these materials in any of a number of properties, including, for example, strength, oxidation resistance, and creep resistance.
Many ceramic materials easily surpass metals in certain high-temperature properties, and therefore offer a potential solution to the limitations of alloys noted above. Ceramics in general are stronger and lighter than high temperature alloys, and resist environmental attack and creep much more effectively. However, ceramic materials have seen relatively little use in many engineering structural components due to their low tolerance for damage. Ceramics tend to be brittle and highly susceptible to rapid catastrophic failure when overloaded, particularly in situations where the ceramic contains mechanical damage in the form of cracks, voids, porosity, or other discontinuity. Brittle materials like ceramics tend to fail with very little to no plastic (permanent) deformation, and the energy required to effect a complete fracture, a quantity often referred to in the art as “toughness,” is comparatively low. Metals and alloys, on the other hand, generally require a comparatively high amount of energy before failing because they exhibit significant amounts of plastic deformation, which discourages formation of cracks and voids, blunts existing crack tips, and otherwise accommodates damage in a way that forestalls catastrophic failure. Materials with high toughness tend to tolerate damage to a much larger extent than brittle materials, due to their ability to “absorb” higher amounts of energy before failing. To be useful, materials that take advantage of the benefits offered by ceramics must also possess some mechanism for enhancing overall toughness and damage tolerance.
One of the most commonly used strategies for achieving the needed balance of strength with toughness in materials incorporating ceramics is the development of a composite material, where multiple materials are combined in a fashion to optimize their advantages while minimizing their disadvantages. Several classes of composite materials have been developed to exploit ceramics. For example, metal-matrix composites include a tough, ductile metal, such as an aluminum or nickel alloy, into which is included a hard, strong, but brittle ceramic that reinforces the softer metal. The incorporation of the ceramic boosts the strength of the composite, while the presence of the ductile metal matrix maintains requisite levels of toughness and damage tolerance. In metal matrix composites, then, the mechanism used to absorb stress and thereby enhance toughness is the plastic deformation of the metal matrix.
Ceramic matrix composites do not include a tough metal phase in the matrix and thus generally employ a different toughening mechanism than metal matrix composites. For instance, in fiber reinforced ceramic matrix composites, an interfacial layer of material may be engineered to be weaker than the respective materials comprising the fiber and the matrix. In such situations strain energy may be absorbed, and failure delayed, by the formation and propagation of multiple small cracks along the fiber interfaces, by frictional sliding of the fiber within the matrix, and other alternative failure modes, instead of the formation and rapid, catastrophic propagation of one large crack as is commonly observed in monolithic ceramics. Ceramic matrix composite materials thus attempt to derive toughening in the absence of plastic deformation through the incorporation of failure mechanisms that allow for a slower, more incremental failure.
Although conventional ceramic matrix composites (CMC's) have shown improvements in toughness and damage tolerance over monolithic ceramic materials, issues remain that detract from the ability to fully capitalize on the benefits offered by ceramic materials. Composite materials in general are mixtures that generally perform only as well as the worst performing constituent in the mixture. For example, poor oxidation resistance in fiber materials results in poor oxidation resistance for the entire composite, because the preferential degradation of the reinforcing fibers has a major effect on the properties of the overall material. Clearly, there is a need for improved materials with high temperature capability and adequate damage tolerance to survive demanding applications.